The legal argument for Willie Pete as a weapon

The maligned white phosphorous sits in a weird, hard to define spot in the military ecosystem. It's part chemical weapon, which would make it illegal, but also part incendiary weapon, which is frowned upon but generally legal. It can be used for marking positions or as a smoke screen, but anyone caught in the smoke risks being burned to the bone.

White phosphorous, often known by the nickname "Willie Pete," is possibly one of the oddest and most controversial weapons on military frontlines, including in American units. Its use as a chemical weapon is banned, but its use as an incendiary weapon is simply limited, and use as a signaling device is fine.

Recent use of phosphorous weapons in Israel has brought this weird dynamic back to the forefront. And the U.S. came under fire for using phosphorous rounds in Iraq. So, what's going on? And is everyone involved now a war criminal?

U.S. Air Force drops a white phosphorous bomb on a Viet Cong position in 1966.

(U.S. Air Force)

First, let's look at why some weapons are illegal, especially chemical weapons. Chemical weapons work by interrupting human processes, some via very gruesome means. Mustard gas causes extreme respiratory irritation, sometimes to the point that those hit by it will develop fatal lung infections. Sarin gas can cause muscle convulsions, paralysis, and respiratory arrest. Both can permanently disfigure people.

In other words, gruesome ways to be wounded or killed.

As a chemical weapon, phosphorous can be released as a gas that is breathed in by the enemy, burning the insides of their lungs and killing them by cooking them from the inside out. Or, it can be introduced into enemy water supplies to poison them. It's illegal to use phosphorous in either of these ways.

But phosphorous is a peculiar beast because, while there are no legally accepted military uses for sarin or mustard gas, there are accepted uses of white phosphorous, because it can also burn people externally or its white smoke can be used to screen troop movements or mark battlefield locations.

The chemical burns at about 86 degrees Fahrenheit. And, when burning, phosphorous emits 5,000 degrees of heat. So, it can spontaneously combust on a warm day, and it can easily sustain its own reaction once it gets going. If it's cold outside, then even a small charge in an artillery shell can ignite the reaction.

Once it's burning, phosphorous emits clouds of thick smoke. For infantry and other maneuver troops attacking an enemy position, that means phosphorous smoke can block the view of defenders trying to kill them. This use of phosphorous is completely legal. It can also be used to mark enemy positions which, again, is completely legal.

(In fact, the 1980 treaty limiting incendiary weapon use specifically does not include weapons used predominantly for marking targets or screening movement.)

Shells from M777A2 155mm Howitzer cannons rain white phosphorous on a target during a four-day, live-fire exercise following the conclusion of Talisman Saber 13 in Australia on Aug 3, 2013.

(U.S. Marine Corps Sgt. Paul Robbins Jr.)

But if you release still-burning phosphorous into the air and get that onto people, then it's extremely dangerous. Phosphorous, again, will continue burning as long as it's exposed to oxygen and above 86 degrees. So, if a chunk lands on a person's shoulder, it will stay above 86 degrees and will keep releasing 5,000 degrees of heat until it runs out of fuel or is drowned in water or mud.

But even drowning phosphorous won't work long-term in human skin, because it will re-ignite from the body heat the moment the water stops flowing. So, in Vietnam, American troops learned to cut the chunks of phosphorous out with knives if any friendlies were hit.

This use of phosphorous is legal, as long as the shooter takes "care" to prevent exposing civilians to the weapon.

And this is the thing that some groups will point to as insane. If it's illegal to use it as a chemical weapon, how can you use the chemical as a weapon without it being a chemical weapon?

Well, first, everything is a chemical, and pretty much all weapons that aren't iron or stone rely on chemical reactions of some kind. Bombs are explosive chemical reactions. Napalm and other incendiary weapons rely on chemical reactions that release a lot of heat, burning the flesh of enemy troops. It's not a chemical reaction that is banned, or the release of heat. Chemical weapon laws really only apply to those weapons which directly interact with the target's cells.

But heating the cells up, as you would with napalm, is legal.

And that's how white phosphorous, as an incendiary weapon, works. It's stored safely encased, then fired against an enemy, exposing it to the air and igniting it in the process. Once the burning phosphorous hits enemy troops, it sears them. A World War II test of phosphorous smoke screens found that, when fired against mock German defenders, the smoke screen would kill or seriously wound 40 percent of the defenders before the U.S. infantry arrived to fight them.

War Dept Film Bulletin 55 White Phosphorus VS High Explosive 1943 (full)

And that's why, as long as the weapon is legal in any context, there will be an incentive for commanders to use it. Without overhead cover, 40 percent of the defenders could be knocked out by the smoke screen. By the smoke screen. High explosive mortar rounds used in the same World War II test generated only 24 percent casualties.

Remember, the point of war is to force an enemy into submission to achieve some political goal. It's gruesome, but it always includes humans killing humans, and explosions and burning are accepted methods of killing each other in war.

And so, the question that will confront investigators looking into Israel's actions will be, "How was the weapon used? And did it cause undue damage to civilians?" Those are the same questions they would have to look at if a bomb was dropped on a church or hospital.

Was this a valid military act, or maybe a valid act that went awry? Or was a commander deliberately harming civilians?